Classes and a friend function between them

Question

I am learning class and friend function. I want the function eat() from the class Farm to access the variable price from the class Bull and change the price value. So I need to implement a friend function. I have problem to use function as I do not understand how to use it. I got thoses errors:

gcc friendFClass.cc -lstdc++ -o friendFClass 
friendFClass.cc: In member function 'void Farm::eat(Bull)':
friendFClass.cc:43: error: 'class Bull' has no member named 'eat'
friendFClass.cc: In function 'int main()':
friendFClass.cc:54: error: 'class Bull' has no member named 'eat'

for this program:

#include <iostream>

class Bull; // declaration not full of Bull

class Farm { // declaration full of Farm

    double sizeFarm;
    double massBull;


    public:
    void eat(Bull eatingBull); // Ok Bull was declared 
    void init(double size, double mass) {
        this -> sizeFarm = size;    
        this -> massBull = mass;
    }
    void print() {
        std::cout << "size of the Farm: " << sizeFarm << "\nmass of a bull: " << massBull << std::endl;
    }

};

class Bull {

    int eatBullThisMonth;
    int price;

    friend void Farm::eat(Bull eatingBull); // Ok Farm was full deblared

    public:
    void init(int price) {
        this -> price = price;
    }
    void print() {
        std::cout << "Number of bull for the month: " << eatBullThisMonth << std::endl;
        std::cout << "Total price : " << price << std::endl;
    }

};

void Farm::eat(Bull eatingBull) {
    int pPrice = 12 * eatingBull.price;
    eatingBull.eat(pPrice);
    eatingBull.print();
}

int main() {
    Farm delacroix;
    Bull marguerite;

    delacroix.init(1260, 123);
    marguerite.init(159);

    marguerite.eat(); 

    return 0;

}

I do not know where to define the friend function, inside or outside the class farm and how to define it.
Thank you.

`eatingBull.eat(pPrice);` <--- What are you trying to do here? Class `Bull` has no function `eat`. — Yksisarvinen, Sep 21 '18 at 11:48
I want with a function from Farm (eat()) to change the value of price. — ripeisripe, Sep 21 '18 at 11:51
you are calling for a `eat(int)` function which is clearly not defined. — Mert Can Ergün, Sep 21 '18 at 11:51
Hmm, the error issued by the compiler `'eat' is not a member of Bull` is clear. BTW what output do you expect? — Jabberwocky, Sep 21 '18 at 11:52
BTW: `friend void Farm::eat(Bull eatingBull);` is pointless as `Farm::eat` is public anyway. — Jabberwocky, Sep 21 '18 at 11:55
@ripeisripe please [edit] your question and clarify what the program is supposed to do according to the comments. — Jabberwocky, Sep 21 '18 at 12:05
The code seems to work now as I think it will be. Can you take a look at it @Jabberwocky please. Thank you all. — ripeisripe, Sep 21 '18 at 12:29
@ripeisripe Don't answer your own question by editing it as it makes no more sense now. You can [answer] your own question. Please read this: [tour]. An no, I don't take a look. If you have another question, ask another question. — Jabberwocky, Sep 21 '18 at 12:30

score 0 · Answer 1 · answered Sep 21 '18 at 11:57

0

Declaring a function as friend in another class, means it can access its private members, not that it inherits the method. In this case it means that Farm::eat has access to Bull private members, but Bull doesn't inherit Farm::eat.

answered Sep 21 '18 at 11:57

Guillem Castro

271
1
5

score 0 · Answer 2 · answered Sep 21 '18 at 12:43

Problem is ok now as it was a misuse of function and misuse of call of a function. Here is the right code.

#include <iostream>

class Bull; // declaration not full of Bull

class Farm { // declaration full of Farm

    double sizeFarm;
    double massBull;


    public:
    void eat(Bull eatingBull); // Ok Bull was declared 
    void init(double size, double mass) {
        this -> sizeFarm = size;    
        this -> massBull = mass;
    }
    void print() {
        std::cout << "size of the Farm: " << sizeFarm << "\nmass of a bull: " << massBull << std::endl;
    }

};

class Bull {

    int eatBullThisMonth;
    int price;

    friend void Farm::eat(Bull eatingBull); // Ok Farm was full deblared

    public:
    void init(int price) {
        this -> price = price;
    }
    void print() {
        std::cout << "Number of bull for the month: " << eatBullThisMonth << std::endl;
        std::cout << "Total price : " << price << std::endl;
    }

};

void Farm::eat(Bull eatingBull) {
    int pPrice = 12 * eatingBull.price;
    eatingBull.init(pPrice); 
    eatingBull.print();
}

int main() {
    Farm delacroix;
    Bull marguerite;

    delacroix.init(1260, 123);
    marguerite.init(159);

    delacroix.eat(marguerite); 

    return 0;

}

Classes and a friend function between them

2 Answers2